In recent years, plastic lenses have become increasingly used as eyeglass lenses because of their lightweight and unbreakability and, thus, high safety over glass lenses. Consequently, plastic lenses are now required to have the same additional functions as those previously imparted to glass lenses. One of such functions is the polarizing property that allows the lenses to selectively block light reflections from various objects. For example, eyeglasses with polarizing lenses are intended to block the glare reflected off road surfaces and other automobiles to which drivers are exposed during driving, as well as the glare reflected off water surfaces, which poses a problem in marine sports, and the glare reflected off snow surfaces, which is a problem in winter sports. Another function required of plastic lenses is the photochromic property, which refers to an ability of a lens to darken or change its color and reduce light transmission when exposed to high intensity light and fade when in a dark environment. There is also an increasing demand for lenses that have both the polarizing property and the photochromic property. While some glass lenses that combine the photochromic property with the polarizing property are already on the market, plastic lenses that are cost-effective but still exhibit the two properties at a sufficient level have never been put on the market due to technical difficulties.
Traditionally, polarizing plastic lenses have been produced by (1) stamping out a thick polarizing sheet and bending the stamped pieces into lenses (See, for example, Patent article No. 1 below) or (2) laminating a polarizing sheet over a lens substrate and further laminating a lens substrate over the polarizing sheet.
The technique (1) is employed in the production of polycarbonate polarizing lenses but is not suitable for the production of correction lenses and high precision plano lenses (See, for example, Patent article No. 1 below). The technique (2) can achieve only low productivity.
To address the above-described problems, several lens production techniques have been developed, including a technique (3) that involves integrating, by injection molding, a plastic to the back of a prebent polarizing sheet to form a lens substrate (See, for example, Patent article No. 2 below), and a technique (4) that involves sandwiching a polarizing sheet with a polymerizable composition and subsequently carrying out polymerization (See, for example, Patent article No. 3 below).
The technique (3) can be used to produce correction lenses and high-precision plano lenses and has thus overcome the drawback of the technique (1). This technique is currently used in the production of polycarbonate polarization lenses (See, for example, Patent article No. 2 below). The technique (4) is of considerable simplicity and potency. In the system of the technique (4) using diethylene glycol bis(allyl carbonate) to serve as a substrate, known is the method using a laminate that comprises triacetylcellulose (which may be referred to as “TAC,” hereinafter) disposed on each side of a polarizing sheet made of a polyvinylalcohol resin.
In comparison with polarizing plastic lenses, photochromic plastic lenses have been produced mainly by “imbibing” technique (See, for example, Patent article No. 4 below). This technique, however, is only applicable to processes that use diethylene glycol bis(allyl carbonate), a compound commonly known as CR-39 (the compound is referred to as “CR-39,” hereinafter).
Thus, several techniques have been proposed that can also be applied to processes that do not use CR-39. Among such techniques are a technique (5) in which a coating layer containing a photochromic pigment is disposed on a lens surface (See, for example, Patent article No. 5 below) and a technique (6) comprising directly adding a photochromic pigment to a lens substrate (See, for example, Patent article No. 6 below). Also proposed is a technique (7), which comprises preparing a laminate with photochromic function, depositing a polymerizable composition on each side of the laminate, and polymerizing the composition to form a lens substrate (See, for example, Patent article No. 7 below).
The technique (5), however, has a disadvantage that the limitation on the coat thickness makes it difficult to achieve sufficient contrast between the colored state and the uncolored state of the lens (See, for example, Patent article No. 4 below). The technique (6) fails to provide sufficient contrast and response time. The technique (7) is still only a concept and has yet to be realized.
To address the above-described problems, techniques for the production of photochromic lenses that rely on injection molding have been proposed. In one such technique, a laminate consisting of a photochromic layer sandwiched between layers of a polycarbonate resin is used to form a photochromic polycarbonate lens. Due to the development of high quality photochromic laminates (See, for example, Patent article No. 8 below), this technique is now being put to practical use.
Conventional techniques also exist for producing plastic lenses that feature both the polarizing property and the photochromic property. One such technique (8) involves laminating a photochromic layer on the surface of a polarizing plastic lens by imbibing technique (See, for example, Patent article No. 4 below).
Some of the techniques of this category are also applicable to processes that do not involve CR-39. Examples include a technique (9) in which a coating layer containing a photochromic pigment is laminated on the surface of a polarizing plastic lens (See, for example, Patent article No. 5 below); and a technique (10) comprising directly adding a photochromic pigment to a lens substrate (See, for example, Patent article No. 6 below).
An effort is also being made to develop laminates that exhibit both high polarizing property and high photochromic property (See, for example, Patent article No. 9).
As far as plastic polarizing lenses are concerned, the technique (4) is only applicable to limited types of lens substrates. In this technique, the polarizing film substrate or the polarizing laminate tends to come off the substrate upon polymerization and similar peeling is likely to occur after molding of the lenses.
Thus, diethylene glycol bis(allyl carbonate), a compound known as CR-39, is currently the only practical material that can be used in the technique (4) to form lens substrates: high index lenses such as those made of thiourethane resins still cannot be made by the technique (4). This is because conditions required for the cast polymerization of, for example, a thiourethane-based polymerizable composition to make high index lenses are significantly stricter than the conditions for polymerization of CR-39. Specifically, performance of the polarizing sheet in the high index lenses may be reduced depending on various polymerization conditions. For example, color variation may occur and the degree of polarization may be reduced. In addition, the polymerizable composition or its reaction product to form the lens may penetrate into the polarizing sheet or polarizing laminate. Furthermore, the adhesion between the lens substrate and the polarizing laminate or the polarizing sheet may be reduced after molding of the lens.
The cast polymerization process in which diethylene glycol bis(allyl carbonate), or CR-39, is used to serve as a lens substrate is the only process currently used for producing photochromic lenses at high productivity.
When the technique (8) is used to produce plastic lenses that have both the polarizing property and the photochromic property, only CR-39 can be used. The technique (9) has a disadvantage that the limitation on the coat thickness makes it difficult to achieve sufficient contrast between the uncolored state and the colored state of the lens (See, for example, Patent article No. 5 below). The technique (10) fails to provide sufficient contrast and response time.